In the state of the art, automatic transmissions, particularly for motor vehicles, comprise planetary gearsets that are shifted by means of friction elements or, as the case may be, shifting elements, such as clutches and brakes, and are usually provided with a starting unit that is subject to a slip effect and optionally provided with a converter lockup clutch, such as a hydrodynamic torque converter or a fluid coupling.
In the Applicant's DE 101 15 983 A1, for example, a multi-step transmission is described that has an input shaft that is connected to an upstream shifting set, an output shaft that is connected to a downstream shifting set, and a maximum of seven shifting elements that can be selectively engaged to create at least seven forward gears without range shifting. The upstream shifting set consists of a shiftable or non-shiftable planetary gearset or a maximum of two non-shiftable planetary gearsets that are connected to each other. The downstream shifting set is configured as a two-carrier, four-shaft transmission with two shiftable planetary gearsets and has four free shafts. The first free shaft of this two-carrier, four-shaft transmission is connected to the first shifting element, the second free shaft to the second and third shifting element, the third free shaft with the fourth and fifth shifting element, and the fourth free shaft is connected to the output shaft. For a multi-step transmission with a total of six shifting elements, the invention proposes that the third free shaft or the first free shaft of the downstream shifting set be additionally connected to a sixth shifting element. For a multi-step transmission with a total of seven shifting elements, the invention proposes that the third free shaft be additionally connected to the sixth shifting element, and the first free shaft additionally connected to a seventh shifting element.
Several other multi-step transmissions are known, for example from the applicant's DE 101 15 995 A1, which discloses four shiftable planetary gearsets that are connected to each other, and six or seven fictional shifting elements, through whose selective locking, a rotational speed of an input shaft of the transmission can be transmitted in such a way to the output shaft of the transmission that nine or eleven forward gears and at least one reverse gear can be implemented. Depending on the gearbox diagram, two or three shifting elements are engaged in each gear, whereby during shifting from one gear into the next following higher gear or the next following lower gear, only one engaged shifting element is disengaged and a previously disengaged shifting element is engaged respectively in order to avoid range shifting.
In addition, the Applicant's generic, previously unpublished patent application DE 10 2005 002 337.1 proposes a multi-step transmission with an input shaft, an output shaft, four individual planetary gearsets that are connected with each other, and five shifting elements in which eight forward gears can be realized without range shifting, i.e., in such a way that when changing from a forward gear into the next following higher or lower forward gear, only one of the previously engaged shifting elements is disengaged, and only one of the previously disengaged shifting elements is engaged. The multi-step transmission also discloses one reverse gear. In all forward gears and in the reverse gear, three shifting elements are engaged respectively. Regarding the kinematic coupling of the four planetary gears sets to each other and to the input and output shaft, it is provided that one carrier of the fourth planetary gearset and the input gear are connected to each other and form a first shaft of the transmission; a carrier of the third planetary gearset and the output shaft are connected to each other and form a second shaft of the transmission; a sun gear of the first planetary gearset and a sun gear of the fourth planetary gearset are connected to each other and form a third shaft of the transmission; a ring gear of the first planetary gearset forms a fourth shaft of the transmission; a ring gear of the second planetary gearset and a sun gear of the third planetary gearset are connected to each other and form a fifth shaft of the transmission; a carrier of the first planetary gearset and a ring gear of the third planetary gearset are connected to each other and form a sixth shaft of the transmission; a sun gear of the second planetary gearset and a ring gear of the fourth planetary gearset are connected to each other and form a seventh shaft of the transmission; and a carrier of the second planetary gearset forms an eighth shaft of the transmission. Relating to the kinematic coupling of the five shifting elements to the four planetary gearsets and to an input and output shaft, it is provided that the first shifting element is arranged in the direction of power between the third shaft and a housing of the transmission, the second shifting element between the fourth shaft and a housing of the transmission, the third shifting element between the first and fifth shafts, the fourth shifting element either between the eighth and second shafts or between the eighth and sixth shafts, as well as the fifth shifting element either between the seventh and fifth shafts or between the seventh and eighth or between the fifth and eighth shafts.
Automatically shiftable motor vehicle transmissions in planetary construction have, therefore, in general, been described many times in the state of the art and are subject to continuous further development and improvement. These transmissions should, therefore, have a sufficient number of forward gears and one reverse gear and a transmission ratio that is very well suited to motor vehicles that have a high total ratio spread and favorable gear steps. In addition, they should allow a high starting ratio in a forward direction, contain a direct gear, and be suitable for use in both private and commercial vehicles. In addition, these transmissions should need only simple construction, in particular a small number of shifting elements, and should avoid double shifts by means of a sequential shift mode, so that only one shifting element is changed during shifting within defined gear ranges.
The present invention is based on the task of proposing a multi-step transmission of the type cited above that has at least eight forward gears that are free of range-shifting and at least one reverse gear which, by using a total of four planetary gearsets, requires the smallest possible number of shifting elements. In addition, the transmission should have a large spread with comparatively harmonic gear-stepping and, at least in the main drive gears, a favorable degree of efficiency, i.e., relatively low drag or cogging losses.